PowerFeed operation of simulated moving bed units: changing flow-rates during the switching interval

Abstract

A possible way to improve the separation performance of simulated moving bed (SMB) units is to change the internal and external liquid flow-rates during the switching period. This operation mode, referred to as PowerFeed, is examined in this work through a model analysis. Similar to the Varicol process, which allows for the asynchronous movement of the ports, the PowerFeed process exhibits more degrees of freedom than the classical SMB process and therefore allows more room for optimization. Using an optimization technique based on a genetic algorithm, all three processes have been optimized for a few case studies in order to determine their relative potentials. It is found that PowerFeed and Varicol provide substantially equivalent performances, which are however significantly superior to those of the classical SMB process.

Introduction

Simulated moving beds (SMBs) have been introduced [1] as a practical implementation of continuous countercurrent units, i.e., the so-called true moving beds (TMBs), in order to solve the problems associated with the movement of the solid. An SMB unit consists of a series of fixed bed columns connected in a circle and divided in four sections by two inlet ports (feed and eluent) and two outlet ports (raffinate and extract), as shown in Fig. 1. The countercurrent movement between the mobile phase and the stationary phase is simulated by synchronously moving the inlet and outlet ports in the same direction of the mobile phase flow. This unit, originally developed for the bulk separation of hydrocarbons, and subsequently extended to sugars, has been more recently applied to a wide range of separations and purifications, particularly in the pharmaceutical and fine chemical industries.

In order to make SMB units more efficient and competitive, several new operation modes have been introduced. These include supercritical fluid SMB [2], [3], [4], [5], temperature gradient SMB [6], solvent gradient SMB [7], [8], [9] and multifraction SMB [10], [11]. The first three improve the separation performance by properly changing the adsorption strength of the solute in the different sections of the unit. This is done by creating along the unit a gradient of pressure, temperature or solvent composition, respectively. Multifraction SMB units are based on the idea of increasing the number of purified fractions leaving the unit by increasing the number of sections, that is, for example in the case of Nicolaos et al. [10], [11] a five+four section unit for a three-component separation.

Another direction which has been taken to improve SMB performance is based on the idea of operating it under more complex forced dynamic conditions. In this context, the SMB unit is not regarded as an approximation (through appropriate discretization) of the TMB unit, but is regarded simply as a unit with many degrees of freedom that can be optimized to improve its performance. The first step in this direction is the Varicol unit [12], [13], where the inlet and outlet ports are shifted asynchronously. This means that the unit is not any longer equivalent to a TMB, but that now it has some more parameters to be optimized, i.e., the switching times of the single inlet and outlet streams. A second possibility has been proposed originally in a patent [14] and more recently by Kloppenburg and Gilles [15] and Zang and Wankat [16], by considering fluid flow-rates changing in time during the switching period. In some sense these two processes can be traced back to a common origin, in that they force a time change during the switching period in either the solid or the fluid flow-rates, which are typically considered constant in TMB units and in the corresponding equivalent SMB units.

In this work, the possibility of changing the fluid flow-rates within the switching period, which we refer to in the following as “PowerFeed” operation, is investigated in detail, using multiobjective optimization technique. The optimal performances that can be achieved by the PowerFeed operation are compared with the corresponding ones given by Varicol and the classical SMB. The aim is to provide a clear picture, although inevitably confined to the cases examined, of the relative potential of these three operation modes.

As mentioned above, in the classical operation mode of the four-section SMB shown in Fig. 1, all the inlet (F and D), outlets (R and E) and internal flow-rates (Q₁–Q₄) are kept constant, while in the PowerFeed operation mode they change in time during the switching period. Actually, since only four of them are independent, in the following we force at most four flow-rates (Q₁, Q₂, F and D) to vary in time, while the other four follow from the mass balance (Q₃=Q₂+F, Q₄=Q₁−D, R=Q₃−Q₄, E=Q₁−Q₂). Although in principle these changes can be continuous in time, in this work, for computational convenience, we consider discontinuous changes, i.e., we assume that each flow-rate can change S times in a switching period t_s, each time taking a constant value. An example of PowerFeed operation is illustrated by the scheme in Fig. 2, where Q₁ is kept constant while Q₂, D and F are forced to take different values in three subintervals of the switching period t_s. As a consequence, E, Q₃, R and Q₄ also change three times in each switching interval.